1. Field of the Invention
The present invention relates to an electronic component comprising an external electrode which is formed on an outer surface of an element body so as to be electrically connected to an internal electrode, and more particularly, it relates to an improvement in structure of such an external electrode.
2. Description of the Background Art
FIGS. 5 and 6 are enlarged sectional views showing parts of multilayer ceramic capacitors as conventional electronic components which are of interest to the present invention. Each of the multilayer ceramic capacitors comprises an element body 1 of ceramic, which is provided therein with an internal electrode 2 of a metal such as palladium, or silver/palladium, for example. An external electrode 3 is formed on an outer surface of the element body 1, to be electrically connected to an edge of the internal electrode 2. The conventional multilayer ceramic capacitors shown in FIGS. 5 and 6 are different in mode of formation of such external electrodes 3 from each other.
In the external electrode 3 shown in FIG. 3, a first layer 4 which is in contact with the edge of the internal electrode 2 and the outer surface of the element body 1 is formed by baking silver paste. A second layer 5 of nickel is formed thereon by wet plating, in order to protect the silver forming the first layer 1. Further, a third layer 6 of tin or solder is formed thereon by wet plating, in order to improve solderability.
In the external electrode 3 shown in FIG. 6, on the other hand, all of first, second and third layers 7, 8 and 9 are formed by dry plating such as sputtering. Such a method of forming an external electrode is described in Japanese Patent Application Laying-Open No. 58-64017 (1983), for example. In more concrete terms, the first, second and third layers 7, 8 and 9 are made of chromium, nickel or nickel/vanadium, and silver respectively. The total thickness of the first, second and third layers 7, 8 and 9 is about 1 .mu.m.
Due to the second and third layers 5 and 6 which are formed by wet plating, however, the multilayer ceramic capacitor comprising the external electrode 3 shown in FIG. 5 encounters the following problems: (1) The plating solution may permeate into the element body 1 of ceramic, to cause insufficient insulation resistance of the as-obtained multilayer ceramic capacitor. (2) The ceramic forming the element body 1 may be electrically reduced during the plating operation. (3) The ceramic forming the element body 1 may be corroded or altered by the plating solution. (4) The plating may grow beyond the first layer 4, to reduce withstand voltage of the multilayer ceramic capacitor.
On the other hand, the multilayer ceramic capacitor shown in FIG. 6 can solve the aforementioned problems. Due to the small total thickness of the external electrode 3 of about 1 .mu.m, however, this capacitor encounters the following problems: (1) The surface of the ceramic forming the element body 1 is inevitably irregularized, and the element body 1 may be partially exposed from the external electrode 3 if the surface is extremely irregularized. (2) The dc resistance of the electronic component tends to increase. This problem is particularly serious in a multilayer inductor. (3) The external electrode 3 may be broken upon supply of a large current. This problem is particularly serious in a multilayer varistor.